In severe hypoxia, SAO2 of 15-18% of the uptake of various amino acids tested decreased from blood to brain. L-tyrosin, L-methionine and D-leucine showed a lower brain uptake than L-alanine, D-leucine and L-histidine. However, L-serine was slightly affected and L-glutamic acid passage across the blood-brain barrier was not affected by hypoxia. Similar reduction of the amino acids brain uptake, except for L-leucine and L-glutamic acid, was observed in severe hypercapnia (pCO2 98-150 mmHg). On the other hand, in hypocapnia, (pCO2 15-18 mmHg) the brain uptake of L-tyrosine and L-methionine appeared little altered, while the other tested substances revealed a normal or increased uptake.